1. Field of Invention
Embodiments of the present invention are related to audio processing, and more particularly to control of a tuner.
2. Related Art
Conventionally, wireless communication reception in mobile devices requires use of high performance receivers that comply with stringent power consumption constraints. A receiver for broadcast services is expected to provide reliable reception under multiple channel impairments such as multi-path fading, Doppler shift, impulse noise, and narrowband noise. A high performance implementation of the receiver can achieve significant improvement of reception range and indoor coverage. Communication and broadcast systems which operate in the VHF/UHF band have particularly high requirements for tuner performance. The VHF/UHF band is populated with an ensemble of high power broadcast transmissions of terrestrial analog TV and radio, as well as aeronautical and governmental transmissions. Receivers operating in these bands are required to handle large dynamic ranges of both a signal of interest and signals in adjacent channels that resides in the VHF/UHF band.
One criterion for a tuner's performance is receiver sensitivities: a minimum receive power at which a digital signal can be reliably decoded in, or an analog signal can be received with some desired signal-to-noise ratio (SNR). Lower receiver sensitivity will enable the receiver to operate at a greater distance from the transmitting antenna. Low receiver sensitivity translates to low noise figure in the tuner.
Another measurement of receiver performance is tolerance to a blocker signal. A blocker signal is a high power signal outside the frequency band of the received channel. Such a blocker signal will be present when the receiver is in proximity to a high power transmitting antenna, which transmits a signal of a frequency band that is close to the received channel frequency. This high tolerance to blocker signals results in high linearity requirements of the tuner's components. The high linearity produces a signal at the output of a receiver component having low non-linear distortions, such as second and third order distortions.
A fundamental tradeoff in any tuner design is power versus noise figure and linearity. For any given tuner architecture, a tuner which is designed for very low noise figure and high linearity will consume much more power then the same tuner with relaxed requirements of noise figure and linearity. Generally, a low noise amplifier (LNA) linearity is inversely proportional to a ratio of the AC signal to the LNA DC bias current. Therefore for a given input radio frequency signal swing to the LNA, increasing the LNA bias current improves linearity (e.g., almost 6 dB every time the bias current is doubled). Furthermore, device speed improves with increased bias current. Whether it is a bipolar or CMOS implementation, the noise figure, generally depends on a ratio of the LNA input signal frequency and a device speed. The smaller this ratio becomes, the better the noise figure is. Therefore, by manipulating the device bias current, both linearity and noise figure can be changed.
Therefore, there is a need for a receiver system comprising a low noise figure and high linearity. There is also a need for a receiver system comprising low average power consumption particularly in mobile and handheld devices.